This invention relates to an improved therma-sensitive transfer sheet (hereinafter referred to as thermal transfer sheet) applicable to recording wherein printing means employing short-period, high-temperature-heating, such as thermal heads, are used.
Heretofore, there has been known a thermal transfer sheet wherein one surface of a base or substrate made of a plastic film is provided with a transfer layer formed thereon and comprising a binder such as wax, and a coloring agent incorporated therein. In recent years, there has been an ever increasing trend towards high-speed printing. When a thermal transfer sheet is used in such a high-speed printing, the back surface of the thermal transfer sheet, i.e., the exposed plastic film surface is heated to its melting point or higher temperature. Consequently, the plastic film undergoes heat fusion to the printing equipment. That is, so-called "sticking phenomenon" is liable to occur. In order to overcome this "sticking phenomenon", several thermal transfer sheets have heretofore been proposed.
For example, Japanese Patent Publication No. 13359/1983 discloses a thermal transfer sheet wherein a heat-resistant protective film comprising at least one material selected from silicone resins, expoxy resins, melamine resins, phenol resins, fluorine resins, polyimide resins and nitrocellulose is provided on the back surface of the thermal transfer sheet. However, must of the resins used in the proposed technology are thermosetting resins, and therefore a heating step for curing is required during the formation of a protective film. Moreover, while the melting point of the protective film is high, the film per se is brittle. This interferes with smooth printing.
We have already proposed that the resins as described above be replaced with thermoplastic resins having an OH or COOH group and that a protective film be formed by adding an isocyanate and the like to the thermoplastic resins and curing the composition (Japanese Patent Application No. 71035/1982). According to this process, curing proceeds at approximately room temperature, and therefore an additional curing step is unnecessary. Thus, the protective film is readily formed. However, the slipperiness of the printing equipments is not yet entirely satisfactory, and there is a problem in that transfer cannot be carried out smoothly.
In general, a heat transfer sheet is mounted on a printing equipment in the rolled form and used in many cases. In such a rolled form, the ink layer of the heat transfer sheet comes into direct contact with the protective film, and therefore these layers may undergo heat fusion during storage at elevated temperatures (so-called blocking phenomenon). Such a blocking phenomenon may also be caused by changes in the environmental temperature and other conditions. Consequently, handling problems arise.
We have carried out studies in order to overcome the drawbacks and difficulties described above. It is an object of the present invention to provide a thermal transfer sheet wherein the sticking and blocking problems which occurred in the prior art can be effectively prevented, whereby printing can be smoothly accomplished.